Recent developments in the field of cellular phones include a system in which a plurality of different transmit-receive signals can be handled with one portable terminal.
For the configuration of such portable terminal, it is usually necessary to have a high frequency switch module comprising discriminating filter 102, low-band high frequency switch 103, and high-band high frequency switch 104 as shown in FIG. 3. A plurality of different transmit-receive signals are input/output to the discriminating filter 102 via common input/output port 101 connected to an antenna (not shown in FIG. 3) corresponding to the transmit-receive signals. The discriminating filter 102 serves to discriminate the transmit-receive signals into high-band transmit-receive signals and low-band transmit-receive signals. The low-band high frequency switch 103 and the high-band high frequency switch 104 serve to discriminate each of the transmit-receive signals discriminateed by the discriminating filter 102 further into transmit signal and receive signal.
A conventional high frequency switch module is configured in that the discriminating filter 102 is a combination of low-pass filter 105 and high-pass filter 106. Also, the low-band high frequency switch 103 and the high-band high frequency switch 104 are configured by switch element 107 such as PIN diode and 90-degree phase shifter 108 which comprises a λ/4 type strip line of transmit signal.
A conventional high frequency switch module is also disclosed, for example, in Japanese Laid-open Patent H11-225088.
In the signal transmitting mode, the transmit signal is input to low-band transmitting port 109 or high-band transmitting port 110 after being amplified by power amplifier (not shown) positioned ahead of the port. However, in low-band transmit signal of low frequency band, the amplification factor of the power amplifier is far greater as compared with that in high-band transmit signal. Accordingly, there arises a problem such that the low-band transmit signal input from the low-band transmitting port 109 leaks into the high-band receiving port 111.
However, in the high-band receiving path from the common input/output port 101 to the high-band receiving port 111, possible elements that may attenuate the leak signal are the high-pass filter 106 configuring the discriminating filter 102, and the 90-degree phase shifter 108 configuring the high-band high frequency switch 104. However, the strip line configuring the 90-degree phase shifter 108 functions as a low-pass filter in high-frequency domain, and also, the frequency band of the low-band transmit signal to be the leak signal is set lower than the frequency band of the high-band transmit-receive signal. Therefore, the high-pass filter 106 configuring the discriminating filter 102 is only element that may attenuate the leak signal.
That is, in this high frequency switch module, it is difficult to prevent the low-band transmit signal input from the low-band transmitting port 109 from leaking into the high-band receiving port 111, that is, to improve the isolation characteristic.